The present invention relates to an anisotropic optical film having therein rectangular-column-shaped structures.
A member having light diffusing property is used in a display device as well as lightening equipment or a building member. Examples of this display device include a liquid crystal display device (LCD), and an organic electroluminescence element (organic EL). A mechanism that the light diffusing member expresses light diffusion is classified into light scattering based on irregularities formed in a surface (surface light scattering), light scattering based on a difference in refractive index between a matrix resin and fine particles dispersed therein (interior light scattering), and light scattering based on both of surface light scattering and interior light scattering. However, such light diffusing members are generally isotropic in diffusing performance. Thus, when the incident angle of light thereinto is somewhat varied, the diffusibility of the resultant transmitted light is not largely changed.
In the meantime, an anisotropic optical film is known, which intensely diffuses incident light with an angle in a predetermined angle range but transmits incident light with an angle outside the range (for example, JP 2547417 B2). This anisotropic optical film is a film obtained by using a linear light source to radiate light onto a sheet-form photosensitive composition layer from above this layer to cure the composition. It is considered that as illustrated in FIG. 10, inside a base of the sheet form, tabular structures 40 different in refractive index from a region around these structures are formed in parallel with each other to be extended consistently with the length direction of the linear light source, which is a source 51, arranged above the anisotropic optical film, which is a film 50, when the film 50 is produced. As illustrated in FIG. 12, a sample 1 (anisotropic optical film) is arranged between a light source not illustrated, and a light receiving unit 3. While the angle of the sample is varied around a central axis that is a straight line L on the front surface of the sample, light is straightly transmitted through the sample. The linear transmittance of the light radiated into the light receiving unit 3 is measurable.
FIG. 11 is a graph showing the incident angle dependency of the scattering property that the anisotropic optical film 50 illustrated in FIG. 10 has, the dependency being measured using the method illustrated in FIG. 12. FIG. 11 is a graph obtained by evaluating the film 50 that is an anisotropic optical film having tabular structures in the same manner as Comparative Examples 2 and 3, which will be described later, have. Its vertical axis represents the linear transmittance (of the film) (i.e., the following light quality when parallel rays having a predetermined light quantity are radiated into the film: the light quantity of parallel rays radiated out therefrom in a direction identical with the incident direction), which is an index showing the degree of scattering (of the film). Its horizontal axis represents the indent angle of the rays. A solid line and a broken line in FIG. 11 show cases of rotating the anisotropic optical film 50, respectively, around the center of an axis A-A (penetrating the tabular structures) and around the center of an axis B-B (parallel with the tabular structures) in FIG. 10. The plus and minus signs of the incident angle denote that directions along each of which the anisotropic optical film 50 is rotated are reverse to each other. According to the solid line in FIG. 11, the linear transmittance is kept small whether the light is radiated in the front direction or in any oblique direction. This matter means that when the anisotropic optical film 50 is rotated around the center of the axis A-A, the anisotropic optical film 50 is in a light scattering state regardless of the incident angle. According to the broken line in FIG. 11, the linear transmittance is small in any direction in the vicinity of 0°. This matter means that also when the anisotropic optical film 50 is rotated around the axis B-B as a center, the anisotropic optical film 50 is in a light scattering state for the light in the front surface direction. Furthermore, in any direction along which the incident angle is large, the linear transmittance is increased. This matter means that when the anisotropic optical film 50 is rotated around the center of the axis B-B, the anisotropic optical film 50 is in a light transmissible state for the light in any oblique direction. This structure can give a property that in transverse directions the transmittance is varied in accordance with the incident angle while in vertical directions the transmittance is not changed even when the incident angle is varied. A curve as shown in FIG. 11, which represents the incident angle dependency of the scattering property (of any member), is called an “optical profile” thereof hereinafter. The optical profile does not directly represent the scattering property. However, when it is interpreted that as the linear transmittance is lowered, the diffuse transmittance is conversely increased, it can be concluded that the optical profile generally represents diffusion property.
About the anisotropic optical film 50, optical properties thereof are determined by the inclination of the film to a normal line of its tabular structures 40. In this case, incident light along directions substantially parallel with the tabular structures 40 is intensely diffused. Light radiated into the tabular structures to penetrate the structures is transmitted without being substantially diffused. Thus, it can be mentioned that the tabular structures 40 are light scattering surfaces.
The nature of this anisotropic optical film 50 depends on the inclination of the tabular structures and the inclination of the incident light. Accordingly, the range of incident angles at which the light is intensely diffused is restricted. Moreover, when the incident angle is varied, the diffusing property of the anisotropic optical film 50 is very sharply changed. Thus, when the optical film 50 is applied to a display device, this property is exhibited as a sharp change of the device in visibility so that the device may give an unnatural impression. In order to solve this problem, given is a method of laminating, onto each other, anisotropic optical films in which their tabular structures are made different from each other in inclination. However, this method has a problem of requiring many costs. Thus, the method is required to be improved. Additionally, in any anisotropic optical film having tabular structures, a light interference (rainbow) is easily generated, and thus this film is required to be improved in visibility.